Synthetic aperture radar (SAR) has been an important tool for remote sensing. The SAR is essentially an active imaging device capable of gathering information relating to phases of reflected signals of a radar beam to form images. The phase of the reflected signals vary approximately quadratically such that the received signal comprises the sum of a number of, approximately linear, chirped signals. The instantaneous bandwidth of a signal is much less than the total bandwidth of the signal.
In spotlight SAR, a radar (mostly airborne) travels along a trajectory while the radar antenna is directed at a fixed region on the ground, forming a “synthetic aperture”—the spatial interval over which data is collected.
Generally, SAR algorithms can be divided into two classes, namely SAR algorithms that remove the quadratic component of the phase variation and SAR algorithms that retain it. Algorithms that remove (dechirp) the quadratic component utilise focusing approximation which would limit the maximum size scene that can be imaged at high resolutions. Algorithms that do not dechirp the data are required to sample the signal fully to avoid aliasing. The higher sampling rate required vastly increases the amount of data to be processed resulting in a corresponding increase in the processing requirements.
Range Migration Algorithm (RMA) is representative of a class of SAR algorithms that uses the actual spherical wave to increase the image quality. The RMA is known as having low processing per pixel and using approximations that are accurate for large high-resolution images. The RMA can perform large-area imaging without breaking a scene into smaller sub-images, and produces well-focused, fine resolution images over a wider area scene.
A feature of RMA is that data are processed using an along-track Fourier transform. As the size of the aperture increases, the bandwidth of the signal to be transformed increases while the instantaneous bandwidth of the signal remains approximately constant.
It is noted that one of the requirements of the conventional along-track Fourier transform is that the data needs to be fully sampled, i.e. the sampling frequency being higher than the signal bandwidth. In order to achieve this, up-sampling of the data is required in cases where the signal bandwidth exceeds the PRF, or down-sampling is limited in cases where the signal bandwidth is significantly larger than the instantaneous bandwidth.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious aspects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive.